1. Field of the Invention
The present invention relates to p-type silicon wafers and to a method for heat-treating the same. More specifically, the present invention relates to p-type silicon wafers having a high resistivity and to a method for heat-treating the same.
2. Background Art
The growing use of portable devices such as cell phones has led to an increase in the demand of p-type silicon wafers having high resistivity (at least 10 Ω·cm) for high-frequency applications. To obtain such p-type silicon wafers having high resistivity, the wafers are prepared at a low dopant concentration. In the case of p-type silicon wafers, doping is carried out using an impurity such as boron at a boron concentration of 1×1015 atoms/cm3 or less.
In p-type silicon wafers having a resistivity of at least 10 Ω·cm, the dopant concentration and the resistivity have a relationship that is almost completely inversely proportional. That is, a decrease in the p-type dopant concentration is accompanied by an increase in resistivity. In p-type silicon wafers which have high resistivity and are used in high-frequency products, it is thus important to have the resistivity in a depth direction be uniform.
A technique for achieving uniform resistivity in a near surface region of a silicon wafer in this way has been disclosed in, for example, the prior-art document cited in Patent Document 1. This patent document discloses a technique for achieving uniform resistivity at or in a vicinity of a surface of a silicon wafer by cleaning the surface with a hydrofluoric acid solution, and then carrying out heat treatment in an argon gas ambient atmosphere.
P-type silicon wafers are heat-treated to reduce defects such as crystal originated particles (COP) in a wafer preparation process, after which they are shipped out. In a vertical furnace of a vertical thermal processing apparatus used to carry out such heat treatment, there is a tendency to occur a buildup of impurities on a wafer boat in a transfer chamber. Such impurities include n-type impurities. N-type impurities are present in an ambient atmosphere of the transfer chamber and end up depositing on the wafer boat in the transfer chamber or on the surfaces of p-type silicon wafers during transfer. Moreover, the longer the wafer boat remains in the transfer chamber prior to heat treatment, the greater the tendency for p-type silicon wafers to be contaminated by n-type impurities. Hence, p-type silicon wafers are contaminated from the surface thereof by n-type impurities. Also, these n-type impurities offset the effects of the boron dopant concentration. As a result, p-type silicon wafers end up having a non-uniform resistivity in the depth direction from the surface to the bulk layer.    Patent Reference 1: Japanese re-publication of International Patent Application, Publication No. WO 01/073838.